Advanced structural ceramic materials have gained the attention of industry by virtue of their superior performance qualities. These qualities, such as superior high temperature strength, high toughness, and resistance to thermal shock and oxidation provide the bases for their potential use in a variety of applications. One particular ceramic, silicon nitride, has been targeted for use in bearing and engine component applications.
One of the more desirable methods for producing silicon nitride ceramics includes hot isostatic pressing ("hipping") in a glass media ("glass hipping"). In the glass hipping process, a preformed powder body ("green body") is first embedded in a glass media (typically a borosilicate glass) and the temperature is raised to soften the glass media so that the green body becomes enclosed in the glass media. When gas-assisted pressure is applied to the glass-enclosed green body and the temperature is raised to a suitable sintering temperature, the glass media prevents the gas from penetrating into the green body, and the green body is densified to a ceramic.
Glass hipping requires substantially lower amounts of sintering aid than gas pressure sintering or pressureless sintering because it utilizes very high (i.e., 200 MPa) pressures which are typically at least about 10 times greater than those used in gas pressure sintering. This is beneficial in that sintering aids frequently degrade both the room temperature and high temperature properties of the ceramic.
Despite the general acceptance of glass hipping, it has also been found that glass hipped silicon nitride ceramics often possess a region of sintering aid depletion near the surface of the sintered ceramic. This region, called a "reaction layer", typically extends about 1000 microns from the surface into the bulk of the ceramic, and is defined by a region having a sintering aid concentration which is no more than 80% of that of the bulk. This reaction layer possesses inferior mechanical structural qualities relative to the internal body. In particular, it is associated with structural defects resulting in decreased rolling contact fatigue ("RCF") life and cosmetic defects. Because of its degradative effects, the reaction layer is typically machined off. However, such machining typically entails diamond machining and so is very expensive, often accounting for over 30% of the total cost of manufacturing the ceramic component.
One proposed solution to the reaction layer problem was advanced in U.S. Pat. No. 4,778,650. This patent disclosed coating ceramic green bodies with a layer of mullite prior to embedding them in the encapsulant glass. However, it has been found that mullite tends to react unfavorably with silicon nitride to form thick (i.e., 1000 micron) reaction layers.
Accordingly, it is the object of the present invention to provide an unmachined sintered silicon nitride ceramic comprising a total sintering aid concentration of between about 1 and 5 weight percent ("w/o"), wherein the ceramic has a reaction layer of less than about 500 microns.